Sectional hollow cathode discharge apparatus



Jan. 27, 1970 J. w. DAVIS ETAL 3,492,525

SECTIONAL HOLLOW CATHODE DISCHARGE APPARATUS Filed March ll, 1968 3Sheets-Sheet l jj f' Jan. 27, 1970 J. w. DAVIS ETAL 3,492,525

SECTIONAL HOLLOW CATHODE DISCHARGE APPARATUS Filed March ll, 1968 3Sheets-Sheet 2.

Jan. 27, 1970 J. w. DAVIS ETAL 3,492,525

I SECTION/XL HOLLOW CATHODE DISCHARGE APPARATUS V Filed March ll, 1968 3Sheets-Sheet 3 United States Patent O U.S. Cl. 313-338 8 Claims ABSTRACTOF THE DISCLOSURE An annular cathode which emits a disc-shaped beam ofelectrons radially inward around its inner periphery, is divided intosegments that are mechanically connected and disconnected to provideaccess to the electron beam focal point.

BACKGROUND OF THE INVENTION Field of invention This invention relates toa novel segmented annular cathode structure which emits a disc-like beamof electrons radially inward toward a focal point, the cathode segmentsbeing -mechanically separable to provide easy access to the cathode Workarea at the focal point.

Description of the prior art Conventional methods of producing electronbeamsv liberate electrons from the surface of a heated cathode bythermionic emission. Electron beams may also be produced bynon-thermionic annular devices such as a hollow cathode and a contouredcathode.

Electron beams are generated by a hollow cathode by the release ofelectrons as a result of both surface emission and the impact ofhigh-energy electrons with background gas molecules within the hollowcathode itself. A novel annular hollow cathode is disclosed and claimedin copending application Ser. No. 417,399 Patent No. 3,381,157 entitledAnnular Hollow Cathode Discharge Apparatus, led Dec. 10, 1964 andassigned to the same assignee.

Contoured cathodes operate in a glow discharge mode in a high pressureregion, and produce electron beams by secondary emission processes suchas from ions striking its surface. Focusing is accomplished bycontouring the surface of the cathode. A novel annular contoured cathodeis disclosed and claimed in copending application Ser. No. 666,230 nowPatent No.v 3,430,091 entitled Contoured Cathode, tiled Aug. 21, 1967and assigned to the same assignee.

Hollow cathodes comprise closed hollow annular rings fabricated fromwire mesh or perforated metal with a circular aperture about its innerperiphery. When the cathode is biased to a high negative potential, withrespect to its surroundings which act as the anode, a glow discharge isinitiated. Under certain combinations of cathode geometry and pressurelevel, a well-collimated disc-like beam of high-current density,high-energy electrons emanates from the hollow cathode aperture and isfocused at the center of the annulus.

The annular hollow cathode and the contoured cathode are used forapplications such as welding, brazing, heat treating, zone melting,fiber drawing and vapor deposition. In these applications, utilizing thecathode of unitary structure, access to the workpiece is obtained onlythrough insertion along the axis.

SUMMARY oF INVENTION An object of the invention is to provide asegmented hollow or contoured cathode which is substantially annular inconfiguration and which may be opened to provide access to the electronbeam focal point.

In accordance with the present invention, an improved segmented hollowcathode is provided wherein the cathode structure has at least twosegments, mechanically or magnetically connected at their ends to enablethe cathode structure to open thereby providing quick access to theelectron beam focal point by a path other than through insertion alongthe axis, as, for example, is essential in order to weld a closedcircular ring. Electrons are emitted from the cathode and areaccelerated radially inward toward a workpiece held at the focal point.Energies are obtained corresponding approximately to the full voltageapplied across the discharge, usually in the kilovolt range,

In a preferred embodiment of the invention, the cathode comprises twosymmetrical segments mechanically connected by a simple hinge alongtheir outer periphery. The mating parts on the opposite side of thecathode are provided typically with pin and pilot hole means for easyalignment.

The invention accordingly provides a segmented hollow or contouredcathode that offers access to the work area at its focal point withoutthe necessity of inserting a workpiece along its axis.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent in the light of the followingdetailed description of preferred embodiments thereof as illustrated inthe accom- -panying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS magnetic fastening means illustratedin-FIGURE 3.

FIGURE 4 is a perspective view of a segmented hollow cathode systemwherein the vacuum chamber has a hinged cover.

DESCRIPTION OF THE PREFERRED EMBODIMENT This invention will be describedby reference to an annular hollow cathode, butv it will be apparent toone skilled in the art that a contoured cathode may be used instead.

Referring generally to FIGURE 1, there is shown a unitary hollow cathodebeam discharge system, and FIG- URE 2 shows the structure of the novalsegmented hollow cathode of this invention.

The annular hollow cathode illustrated in FIGURE 1 is described indetail in copending patent application Ser. No. 417,399, now Patent No.3,381,157.

The annular hollow cathode 20 in FIGURE 1 is fabricated, for example,from stainless steel, from a 10` mil, 40 mesh wire cloth or similarsubstance. Cathodes thus far fabricated have an outer diameter, D0,typically from 1.3 to 4 times the inner diameter Di, but are obviouslynot limited thereto. The height H of the cathode assembly does notappear to be a critical factor; however, the size of the aperture A canbe critical as is explained in copending patent application Ser. No.417,399, now Patent 3 No. 3,381,157. The wall thickness or mesh size donot have an appreciable affect upon the electron beam output. Also, thehollow, solid-wall cathode structure illustrated in FIGURES 1 through 4can be a perforated wall configuration, such as the perforated wallconfiguration illustrated in the above referenced copending application.

In FIGURE 1, the cathode 20, workpiece 22 and associated supports areenclosed in an airtight enclosure 24 which comprises glass or othersuitable material. The cathode is supported by arm 26 which is also thenegative potential lead to the cathode. Arm 26 may be covered by aninsulating material, not shown. Workpiece 22 is held in place by ametallic arm 30 on which are positioned two adjustable clampingstructures 28 and 28. If the workpiece 22 is metallic, it is grounded toact as the anode. If the workpiece 22 is a nonconductor, the workpiecesupport structures 28, 28 and 30 act as the anode.

The enclosure 24 is initially evacuated by means of vacuum pump 36.After evacuation to the proper pressure level, a supply of gas 38 isused to produce a gas atmosphere within the enclosure. The gas can beair, helium, hydrogen, nitrogen or argon, or any gas suitable for theworkpiece.

The theory of operation of the hollow cathode. as presently known, isexplained in considerable detail in copending patent application Ser.No. 417,399, now Patent No. 3,381,157. Basically the operation may becompared with the operation of conventional glow discharges. pared withthe operation of conventional glow discharges. In a conventional glowdischarge, practically all of the potential drop across the dischargeoccurs in a region quite close to the cathode, this drop being known asthe cathode fall. The characteristic thickness of the cathode falldepends on the gas pressure, gas type, cathode material and appliedvoltage. The high potential end of this region can be identifiedvisually by a sharp demarcation between a dark portion of a dischargenear the cathode, called the cathode dark space, and a blight regioncalled the negative glow. Equipotential lines for this distribution areparallel with the cathode. As explained in detail in copending patentapplication Ser. No. 417,399, now Patent No. 3,381,157, the perturbedpotential distribution in the vicinity of the aperture A resembles aconcave lens. The electric field lines normal to the equipotential linesconverge in the vicinity of the aperture. Electrons generated inside thecavity drift toward the aperture in the relatively weak electric fieldtherein. In the aperture region, they are accelerated through the fullcathode fall and thus acquire a highly directed velocity approximatelyalong the field lines. In this manner, the perforated wall hollowcathode forms a highly collimated energetic electron beam.

FIGURE 2 is an illustration of the segmented annular hollow cathode ofthe invention. The cathode 20 is divided into two symmetrical segments30 and 32. A pin and pilot hole arrangement is provided wherein pins 34on one end of segment 30 fit into pilot holes 36 on the correspondingend of cathode segment 32. At their opposite ends, cathode segments 30and 32 have attached hinges 38, 40 respectively. These hinges, whenconnected by a pin 42, enable the cathode segments 30, 32 to be pivotedopen, thereby allowing access to the focus of the electron beam.

FIGURE 2A is a view of the hinge and pin arrangement illustrated inFIGURE 2, and represents only one of many possible mechanical fasteningarrangements anticipated by the invention. In addition, it is notessential to divide the cathode into two equal sections. More than twosections of different sizes are possible. As can be seen in FIGURE 2,pivoting of the cathode sections 30 and 32 about their pivot pointprovided by hinges 38, 40, permits the cathode to be opened, making itpossible to easily place workpiece 22 therein.

Of course, segments 30 and 32 can be connected ly means of pins 34 andpilot holes 36 on each end thereof,

thereby eliminating the hinge. With this arrangement, it

access to the cathode work area.

The embodiment of FIGURES 3 and 3A illustrates another of the manyunlimited possibilities for fastening the cathode segments 30, 32. Inthis embodiment, small permanent magnets 43 are permanently attached tothe ends of each symmetrical segment and when these segments are broughttogether in proper alignment, the magnets 43 securely hold the segments30, 32 in a fixed abutting relationship. Of course, there is apossibility of :omc interaction between stray magnetic fields and theelectron beam. Also the magnetic fastening means could be sensitive totemperature.

FIGURE 4 illustrates a typical segmented hollow cathode system and, ingeneral, comprises a sectional vacuum chamber 44, segmented cathode 20and a workpiece 22. In this embodiment the vacuum chamber 44 has ahinged cover 46 that opens to provide access to the interior of thechamber so that the workpiece 22 can be easily situated therein. Thesegmented cathode 20 is divided into two segments as describedhereinbefore in conjunction with FIGURE 2. The cathode 20 is supportedby arm 48 which is also the negative potential lead. The workpiece 22rests in semicircular depressions 50, 50 in the container 44 and thecover 46 closes and fastens by means of clamps S2, 52. The rubbergaskets 54, S6 and 56 insure that the container 44 is airtight after thecover 46 has been closed and fastened. Seals 4S and 47 across the endsof workpiece 22 insure that the workpiece is airtight. The chamber isevacuated through pumping line 60 and gas feed line 62 provides meansfor introducing gas into the chamber as discussed in conjunction withcopending patent application Ser. No. 417,399, now Patent No. 3,381,157.

As indicated previously, the contoured cathode described in copendingapplication Ser. No. 508,314 may also be segmented in the mannerdisclosed herein by those skilled in the art without departing from thescope of this invention. Furthermore, it is apparent that the segmentedcathode need not be fully annular, but may be of any closed or partiallyclosed geometry.

It is apparent that shielded annular hollow cathode configurations suchas disclosed in copending application Ser. No. 508,201, Patent No.3,454,812 entitled Mounting for a Glow Discharge Cathode and filed Nov.17, 1965, may also ybe adapted to lbe segmented. In a shieldedconfiguration the cathode is surrounded by a conducting or insulatingshield spaced a short distance therefrom for the purpose of inhibitingthe formation of a plasma adjacent the cathode walls. The cathode may bea perforated or solid wall device. With shielded configurations it isdesirable to locate the hinge on the shield and to mount the cathodesegment to the shield by well-known techniques. This latter approach iseffective to maintain the cathode free from protuberances.

Although the invention has been shown and described Iwith respect topreferred embodiments thereof, it should be understood by those skilledin the art that the foregoing and other changes and omissions in theform and detail thereof may lbe made therein without departing from thespirit and scope of the invention, which is to be limited and definedonly as set forth in the following claims.

Having thus described typical embodiments of the invention, that whichis claimed as new and to be secured by Letters Patent of the UnitedStates is:

1. A cathode device for producing an electron beam which comprises:

a segemented cathode structure having a substantially closed perimeter,each of the segments comprising a cathode element adapted to emitelectrons therefrom toward a focal point,

and connecting means for joining the cathode segments to each other inan abutting relationship, the connecting means being adapted forconnection and disconnection to provide access to the electron beamfocal point.

2. The cathode device of claim 1 wherein the connecting means ismagnetic.

3. The cathode device of claim 1 wherein the connecting means comprisesa hinge and pin pivotally connecting adjoining cathode segments alongtheir outer periphery so that they may be pivoted open.

4. A cathode device as in claim 1 wherein the connecting means comprisesa pin extending from the end of a rst cathode segment and a pilot holein the adjoining end of a second cathode segment and adapted so that thefirst and second segments can Ibe connected by the insertion of the pinin the pilot hole.

5. The cathode device of claim 1 wherein the cathode is a hollowcathode.

6. A cathode device as in claim 1 and including a segmented shieldsurrounding said cathode, said connecting means joining said shieldsegments in abutting relation.

7. Apparatus as in claim 1 wherein said cathode is a contoured cathode.

y8. An apparatus for generating an electron beam which comprises:

a segmented -cathode structure having a substantially closed perimeter,each of the segments comprising a cathode element adapted to emitelectrons therefrom toward a focal point;

connecting means for joining the cathode segments to each other in anabutting relationhsip, the connecting means being adapted for connectionand disconnection to provide access to the electron beam focal point;

enclosure means surrounding the cathode structure, the enclosure meansproviding an access to the cathode structure and having sealing means toproduce an airtight seal about the enclosure means;

means to produce a vacuum in the enclosure; and

means for causing the cathode structure to emit therefrom a planecollimated uninterrupted sheet of electrons` References Cited UNITEDSTATES PATENTS 1,727,373 9/1929 Macksoud 313-338 X 2,748,310 5/1956Agule 313-247 3,381,157 4/1968 Ferreira 313-339 X 3,406,307 10/1968Dugdale 313-339 X 3,418,515 12/1968 Maskell 313-339 X JOHN W. HUCKERT,Primary Examiner 5 ANDREW J. JAMES, Assistant Examiner

